Cellular communication networks evolve towards higher data rates, together with improved capacity and coverage.
In this context, TTI bundling is an advanced technique to improve uplink coverage, and especially at the cell edge and/or at poor radio conditions.
FIG. 1 is a schematic diagram illustrating an example of a base station 10 and associated User Equipment (UE) 20 in a radio communication system. The UE 20 is generally power-limited, and when the radio conditions are not optimal and path loss is high, there is a significant risk for packet errors and/or packet losses.
TTI bundling, which is also referred to as subframe bundling, involves uplink transmission of a packet multiple times in consecutive TTI intervals, also referred to as subframes, without waiting for any transmission acknowledgement (ACK/NACK) feedback. This improves the chances of successful uplink transmission from User Equipment, especially power limited UEs.
Long Term Evolution (LTE) systems, for example, performs resource allocation to UEs adaptively by the concept of fast scheduling, normally taking into account the instantaneous traffic pattern and radio propagation characteristics of each UE. Assigning resources in both downlink (DL) and uplink (UL) is performed in the scheduler situated in the eNodeB (eNB).
In LTE, as an example, packets are delivered using the Internet Protocol (IP). This means that also traditionally circuit switched services such as voice conversation will make use of fast scheduling. This is called Voice over IP (VoIP). 3GPP has standardized a mechanism called TTI Bundling, also called “subframe bundling” in some 3GPP specifications, specifically for UE's using VoIP while they are limited by their transmission power to the extent that they need to segment their IP packets several times. When this mechanism is used the whole packet is sent four times in consecutive TTIs. This effectively increases the transmitted power used four times. It is not favorable to let UEs that do not segment their packets use TTI Bundling because for these UEs TTI Bundling causes a four times increased Uplink Shared Channel (UL-SCH) usage. Thus, in realistic scenario's there will be a mixture of UEs using TTI Bundling and UEs not using TTI Bundling. The mechanism that determines which UEs shall be configured for TTI Bundling and which UEs shall operate with normal Hybrid Automatic Repeat reQuest (HARQ) operation is a crucial part of the TTI Bundling implementation.
The drawbacks of not using TTI Bundling while this is beneficial are:                Reduced VoIP quality.        Inefficient usage of UL-SCH and control channel resources.        
The drawbacks of configuring a UE for TTI Bundling while this is not necessary are:                Inefficient UL-shared channel (SCH) usage: More UL-SCH resources are used than required.        Inefficient usage of processing resources in the eNB as well as resources on the air interface due to the Radio Resource Configuration (RRC) reconfiguration that is required to enable/disable TTI Bundling.        Increased interference due to the usage of TTI Bundling.        
It is thus desirable to find an efficient way of determining whether TTI bundling should be enabled or disabled for the UE(s).